added cmfd section to theory which includes notation section
added support for tikz extension in sphinx and included first part of theory section
added calc of macro xs section
added more content up to toy problem
added last toy problem
removed $ from docs
Merge branch 'develop' into cmfd-theory
Merge branch 'develop' into cmfd-theory
Merge branch 'develop' into cmfd-theory
addressed @paulromano comments
Check for negative values when calculating cross sections from URR probability
tables.
Merge pull request #327 from bhermanmit/cmfd-doc

CMFD Documentation
Fix typo
Merge pull request #328 from paulromano/urr-zeros

Check for negative values when calculating cross sections from probability tables